In the Session of 1867-8 I communicated to this Society a paper on the Dissipation of Energy, of which only a very brief abstract was published in the Proceedings. The main feature of that paper was the suggestion, as at least a valuable working hypothesis, that even in cases of the steady motion of heat, electricity, &c., the unexhausted energy is probably as small as possible, consistently with the conditions of each form of experiment.

The presentation to the Royal Society of Edinburgh of results relating to the decennial period, derived from observations of magnetic declination made during nearly a quarter of a century at Trevandrum, has seemed to me a favourable occasion for a determination of the mean duration of this period. Upon the explanation of the decennial variations depends the solution of several important problems in solar and terrestrial physics, and the first step towards this result is to ascertain the true mean duration of the period. Two markedly different results have been obtained, each of which has been accepted by men of the highest scientific reputation.

Although the subject of the following communication has of late years attracted a great deal of attention among the public generally, it may, nevertheless, be well for me to preface my statements by a few elementary remarks.

It is well known that organic substances, when left exposed under ordinary circumstances, undergo alterations in their qualities. For example, an infusion of malt experiences the alcoholic fermentation; a basin of paste prepared from wheaten flour becomes mouldy; or, again, a piece of meat putrefies when so treated. The microscope shows that each of these changes is attended by the development of minute organisms. In the fermenting sweet-wort the yeast which falls to the bottom of the containing vessel is found to consist of budding cells, constituting the yeast-plant, Torula Cerevisiæ, represented in Plate XXII. fig 2. In the mouldy paste the blue crust which is the most frequent appearance, owes its colour to the spores of a species of filamentous fungus, Penicillium Glaucum, the commonest of all moulds, of which fig. 1 in Plate XXII. represents a pencil of fructifying threads; and the putrid flesh will be probably found teeming with bodies which, in the most typical form, consist of two little rods, connected endways as by a joint, such as are seen at a, fig. 3, Plate XXII., characterised by astonishing powers of locomotion, and, from their rod-like form, termed Bacteria.

Sensory nerves are divided into two classes, those of general sensibility and those of special sense. The nerves of general sensibility are distributed to the skin, muscles, or viscera, and convey influences to the brain which give rise to sensations of touch, heat, &c., or to those vague sensations, not definitely localised, which we include under the name of the muscular sense. The nerves of special sense are endowed with special and individual physiological properties. When a nerve of this order is irritated in any way, either by mechanical, chemical, or electrical stimuli, an influence is conveyed to the brain which gives rise to the same kind of sensation as that produced by the normal stimulus on the terminal organ. For example, pressure on the eyeball, as shown by Newton and Young, electrical stimulation by a continuous current, as demonstrated by Pfaff, Helmholtz Ritter, Purkinje, Du Bois-Reymond, and Schelske, produce many of the phenomena of vision, including not only the perception of light, but the perception of various colours and tints. But while this is the case, it is equally certain that each terminal organ responds to its normal stimulus. Thus the retina, though capable of stimulation by pressure or electricity, is specially fitted by its histological structure for the reception of those minute vibrations of the ether which constitute light. But while the terminal organ is capable of receiving a most delicate action of the normal stimulus, the nerve in connection with it is not so affected. For example, although the retina is affected by light, the optic nerve is not so, as may be proved by Marriotte's well-known experiment, by which it may be demonstrated that when the image of an external object falls on the entrance of the optic nerve, there is no corresponding sensation. The nerve is thus insensible to the normal stimulus of the sense organ, the retina.

I. The Parallel Roads of Lochaber have presented to geologists a problem, which is still unsolved. Dr Macculloch, about sixty years ago, when President of the Geological Society of London, first called attention to these peculiar markings on the Lochaber Hills, by an elaborate Memoir afterwards published in that Society's Transactions. He was followed by Sir Thomas Dick Lauder, who in the year 1824, read a paper in our own Society, illustrated by excellent sketches. His paper is in our Transactions. The next author who attempted a solution was the present Mr Charles Darwin. He maintained that these Roads were sea-beaches, formed, when this part of Europe was rising from beneath the Ocean. He was followed by Professor Agassiz, Dr Buckland, Charles Babbage, Sir John Lubbock, Robert Chambers, Professor Rogers, Sir George M'Kenzie, Mr Jamieson of Ellon, Professor Nicol, Mr Bryce of Glasgow, Mr Watson, and Mr Jolly of Inverness. Sir Charles Lyell, though he wrote no special memoir, treated the subject pretty fully in his works, giving an opinion in support of the views of Agassiz.

In the month of August 1872, Professor Turner suggested as a subject of investigation the structure of the ovary, and development of the ova, more especially with reference to the recently published observations of Waldeyer.

In the month of April 1874, I handed in to the Medical Faculty of the University of Edinburgh, as my graduation thesis for the degree of M.D., an essay entitled “Contributions to the Normal and Pathological Anatomy of the Ovary and Parovarium.”

In March 1869 I communicated to the Society a paper entitled “Motion of a Palladium Plate during the Formation of Graham's Hydrogenium,” which appears in the Proceedings for Session 1868-69. When engaged with this subject many points of interest regarding the behaviour of palladium containing occluded hydrogen, suggested themselves for investigation, and in concluding the paper I remarked “ that careful determinations must be made of the electromotive force, latent heat, &c., of hydrogenium ” before we could arrive at any conclusion regarding the condition of the absorbed hydrogen. Subsequently Professor Tait made a series of determinations on the “ Electrolytic Polarisation of Palladium Electrodes,” devising a new and ingenious method for the purpose. Although at different times subsequent to my first communication, the problem of determining the physical constants of hydrogenium recurred, as my attention was in the meantime directed to the specific heat of carbon at high temperatures, no progress was made with the investigation until September 1872, when the results of my preliminary experiments were communicated to the “ Philosophical Magazine,” under the title, “ Note on the Specific Heat of Hydrogenium.” In that note it is stated that by means of a specially constructed calorimeter the specific heat of hydrogen in palladium is found to be 3.1 per atomic weight, nearly identical with that of gaseous hydrogen. The present paper deals with some of the physical constants of hydrogenium, more especially with the specific gravity, specific heat, and co-efficient of expansion.

Concerning the affinities and systematic position of this very remarkable Devonian fish, there has hitherto prevailed very great uncertainty. The two original specimens, discovered by Mr C. W. Peach, in the Old Red Sandstone of John O’Groat’s, Caithness, and described by Sir Philip Egerton, left us in complete ignorance as to the osteology of the head and the dentition, while the evidence they afforded as to the structure of the pectoral fins was by no means so clear as might have been wished for. To quote from Sir Philip's description:—“ The bones of the head, with the exception of a small fragment of the operculum, are wanting, but the impressions left upon the matrix show that they were sculptured in rather a bold pattern, not unlike the ornament on some of the cranial bones of some of the Holoptychii, and consequently differing in this respect from the corresponding parts in Dipterus. The pectoral fins are very indistinctly seen. They appear to have had a short obtuse lobe forming the base, and extending therefrom a set of numerous fin-rays more elongated than those forming the pectoral fin in Dipterus.” To Dipterus, however, in Sir Philip Egerton's opinion, its affinities pointed, as far as could be gathered from the structure of the body as displayed in the specimens, his description concluding as follows:—“ The absence of all evidence as to the dental apparatus of Tristichopterus is much to be regretted. On other points the affinities between this genus and Dipterus are so striking that they cannot be classified in separate families. Accordingly he assigned to Tristichopterus a place along with Dipterus in the family of “Cœlacanthi,” the term being used in its former extended sense, not as now restricted to the peculiar genera Cœlacanthus, Undina, Holophagus, and Macropoma.

Of the periodical variations of atmospheric pressure the best marked are the diurnal oscillations which in tropical and sub-tropical regions are among the most regular of recurring phenomena. In higher latitudes these oscillations become from day to day gradually more and more masked, owing to the frequent fluctuations to which atmospheric pressure is there subjected. If, however, hourly observations continue to be regularly made for some time, the diurnal oscillation becomes apparent in the averages deduced from them.

In referring to the tendency to bilateral ramification on the branches and even on the main stem in Cupressineæ, such as Cupressus, Thuja, and Libocedrus, Professor Sachs, in his Text-book (Bennett's translation pp. 444–5), thus expresses himself: “Branch systems of three or four orders of shoots are developed in one plane in such a manner that a system of this kind assumes a definite contour, and somewhat the appearance of a pinnate leaf. In Taxodium the foliage-leaves are formed in two rows on slender branches a few inches in length; in T. distichum these fall off in autumn, together with their leaves, thus presenting a still greater resemblance to pinnate leaves.”

At the meetings of the Society on February 1 and 15, 1875, I exhibited a number of photographs which had been executed for me in my laboratory by Mr A. Matheson, one of my students who has acquired great skill in working with very sensitive films of collodion. My object in advising him to undertake this work was to discover, if possible, the cause of the peculiar zig-zag form which electric sparks, and specially those of a Holtz machine, always show in ordinary air. At the desire of the Council, a selection of a few of the more interesting of these photographs has been printed (by Mr Dallas, F.R.S.E.) for the Transactions from the original glass negatives, by what is commonly called the carbon process.